util/disk_cache: don't fallback to an empty cache dir on evict

If we fail to randomly select a two letter cache dir, don't select
an empty dir on fallback.

In real world use we should never hit the fallback path but it can
be hit by tests when the cache is set to a very small max value.

Reviewed-by: Grazvydas Ignotas <notasas@gmail.com>

src/util/disk_cache.c

@@ -576,14 +576,13 @@ unlink_random_file_from_directory(const char *path)
    }
 
    unlink(filename);
-
    free (filename);
 
    return sb.st_size;
 }
 
 /* Is entry a directory with a two-character name, (and not the
- * special name of "..")
+ * special name of ".."). We also return false if the dir is empty.
  */
 static bool
 is_two_character_sub_directory(const struct dirent *entry, const char *path)
@@ -594,15 +593,37 @@ is_two_character_sub_directory(const struct dirent *entry, const char *path)
 
    struct stat sb;
    int res = stat(subdir, &sb);
+   if (res == -1 || !S_ISDIR(sb.st_mode)) {
+      free(subdir);
+      return false;
+   }
+
+   if (strlen(entry->d_name) != 2) {
+      free(subdir);
+      return false;
+   }
+
+   if (strcmp(entry->d_name, "..") == 0) {
+      free(subdir);
+      return false;
+   }
+
+   DIR *dir = opendir(subdir);
    free(subdir);
 
-   if (res == -1 || !S_ISDIR(sb.st_mode))
+   if (dir == NULL)
-      return false;
+     return false;
 
-   if (strlen(entry->d_name) != 2)
+   unsigned subdir_entries = 0;
-      return false;
+   struct dirent *d;
+   while ((d = readdir(dir)) != NULL) {
+      if(++subdir_entries > 2)
+         break;
+   }
+   closedir(dir);
 
-   if (strcmp(entry->d_name, "..") == 0)
+   /* If dir only contains '.' and '..' it must be empty */
+   if (subdir_entries <= 2)
       return false;
 
    return true;